1. Field of the Invention
The invention relates to a silane-crosslinked polyolefin insulated wire and, in particular, to a silane-crosslinked polyolefin insulated wire that the silane-crosslinked polyolefin is prepared by a crosslinking reaction using a new crosslinking promoter instead of an organotin compound that may cause adverse effect as an environmental hormone.
2. Description of the Related Art
A widely used electric wire is provided with an insulating cover layer that is prepared such that a graft copolymer obtained by graft-copolymerizing an organic silane compound to a polyolefin such as polyethylene, ethylene-vinyl acetate copolymer and ethylene-propylene copolymer, in the presence of a free radical generator, or a copolymer of polyethylene and vinylsilane compound is molded into a predetermined shape, and is reacted with water in the presence of a silanol catalyst to produce, as the insulating cover layer, the molded material with molecules crosslinked each other.
This crosslinking method called “silane-water crosslinking” is characterized in that a small amount of organic peroxide is caused to act as a grafting initiator on the polyolefin in a processing equipment such as an extruder to graft-copolymerize a silane compound such as vinyl alkoxysilane to the polyolefin, and the molded material discharged from the processing equipment is then exposed to high temperature and high humidity or to hot water to cause the crosslinking reaction.
The crosslinking reaction is completed by the hydrolysis and condensation reaction of alkoxysilane by the aid of a silanol condensation catalyst (typically an organotin compound) that is preliminarily mixed into the molded material or infiltrated through the surface of the molded material. In other words, the reaction is based on that alkoxysilane graft-copolymerized to the polymer is subjected to the hydrolysis and condensation reaction to bond the polymer molecules each other to promote the crosslinking. The crosslinking method is easier and less costly on facility basis and on process basis than a so-called chemical crosslinking method in which only the organic peroxide is used for crosslinking. Therefore, it is the most suitable crosslinking method of a molded material such as an insulating coating of, especially, an electric wire/cable.